The present invention relates to improvements in or relating to Codec negotiation and selection, particularly but not exclusively in a radio access network.
For both, Tandem Free Operation (TFO) and Transcoder Free Operation (TrFO) by using Out-of-Band Transcoder Control (OoBTC), there is a continual effort to avoid speech quality impairment caused by unnecessary transcoding steps along the speech path. In order to achieve this both technologies rely on an exchange of lists of Codec types to be used on each sub-link of the speech path. Tandem Free Operation (TFO) is an in-band procedure used to pass compressed speech through a pulse code modulation (PCM) connection. TFO is applied after call setup and includes the exchange of Codec information between TFO-peers using an in-band protocol. The optimal common Codec is determined according to a predetermined set of rules by both peer nodes. Out-of-Band Transcoder Control (OoBTC) is a procedure between call control nodes (CCN), where a Codec is negotiated using an out-of-band protocol. This is applied before or at call setup and the selection of the optimal Codec is made by the terminating node. There are no fixed or predetermined selection rules.
In the call setup procedure of a call originating from a GSM mobile (also called the Mobile station (MS) or the GSM terminal) the Mobile Switching Center (MSC) server (MSC-S) at the originating side of the call receives information about the terminal supported Codec types of the MS. This information is sent from the MS to the MSC within a direct transfer application part (DTAP) message during the early call setup signaling. This message passes physically through the base station controller (BSC), although the BSC is not functionally involved in the communication. The originating MSC-S then performs Codec negotiation with the peer terminating MSC-S in order to determine an appropriate Codec to be used in the core network. This is generally referred to as the OoBTC procedure and generates the selected Codec (SC). The terminating MSC-S selects the SC, determines an available Codec list (ACL) and sends these (SC and ACL) back in a message to the originating MSC-S. The originating MSC-S then builds a list of possible speech Codec versions in the form of a speech Codec version list (SCVL) which is ordered in accordance with the originating MSC-S preferences. If any of the speech Codec versions are identical or compatible with the SC used in the core network, these are preferred over other Codec versions. This is due to the fact that this allows TFO operation on the GSM signal interface between the BSC and the MSC (A-Interface). The originating MSC-S then enables TFO in a media Gateway (MGW) node that connects to the A-interface. The originating MSC-S also requests channel assignment from the originating BSC so that the BSC can seize the radio bearer. The request includes the SCVL of possible Codec types. At the same time the originating BSC chooses an appropriate Codec type from the list and the core network bearer is seized using the negotiated Codec type (SC). The MSC-S may be notified that the bearer setup procedure has been performed successfully. A first transcoding unit within the originating base station system (BSS) and a second transcoder unit within the originating MGW perform the TFO negotiation. If the Selected Codec used in the core network and the Codec chosen by the originating BSC are compatible then TFO becomes operational and transcoding free operation is achieved at least on the originating side of the call. A complementary series of actions is performed by the terminating MSC, MGW, BSC and MS and if the Selected Codec used in the core network and the Codec chosen by the terminating BSC are compatible, then TFO becomes operational and transcoding free operation also on the terminating side of the call is achieved. If TFO becomes operational on both sides then end-to-end transcoding free operation is achieved, which is the ultimate goal.
If the call setup originates or terminates from different types of radio access network, there are variations of the above described process which may apply. However, there are a number of problems which exist with the above described process. The standardized/specified procedure for call setup requires the originating MSC-S to perform Codec negotiations within the core network prior to communicating an assignment message with the originating BSC. This assignment message is used to request the seizure of the relevant radio channel and can provide a preferred speech version list (PSVL) defining a set of speech Codecs that can be used for the call. Within the list the first Codec is generally the most preferred. The BSC is now free to select any speech Codec from this PSVL. The BSC decision occurs after the Codec is selected in the core network and based on the fact that the BSC does not know about the Codec negotiations or any end-to-end problems in the network. Accordingly, the BSC decision is based simply on a local view. This often results in the situation that the BSC is selecting a Codec that is not compatible with the Codec selected in the core network or in any other (terminating) BSC along the path of the call.
Alternatively, the BSC may select the Codec that is preferred by the MSC-S, but not one that supports TFO or transcoding free operation (TrFO). As a consequence TFO cannot be achieved on the A-interface (in general: on the interface between Core Network and Radio Access Network) and transcoding has to be performed which has negative impact on the speech quality of the call and/or increases the costs of Media Gateway resources.
There are a number of possible reasons why the BSC selects a Codec that is incompatible with the core network (i.e. does not select the first element in the received PSVL), these include:                The transcoding resources for the first Codec are not currently available to the BSC.        The transcoding resources are available but TFO support is not available.        Due to temporarily or locally limited radio channel capacity the BSC may select a half rate channel instead of a preferred full rate channel.        
It is possible to correct for incompatibility and failed TFO setup through a change of the selected Codec in the core network. This can be achieved by means of an OoBTC mid-call modification or negotiation procedure. However, this brings about the following disadvantages:                Changing the Selected Codec in the core network cannot occur without negative impact on the speech connection thereby reducing the overall speech quality of the call.        Any change of the Selected Codec in the core network may trigger a change of the Codec used in the radio access on the other side of the call. In addition the other side of the call has the freedom to reject the new Selected Codec and as such the attempt to change the Selected Codec may completely fail.        Any change of the Selected Codec in the core network may lead to a mismatch towards the Codecs used in the radio access on the other side of the call, thereby reducing speech quality of the call.        Any change in the Selected Codec in the core network requires additional signaling and therefore additional network resources.        An OoBTC mid-call modification or negotiation procedure can be rejected by the terminating side resulting in unnecessary use of signaling resources.        An OoBTC mid-call modification or negotiation procedure may fail completely in mixed vendor or mixed configurations environments.        